1. Field of the Invention
The present invention relates to an idling control device of a spark ignition type internal combustion engine.
2. Description of the Related Art
As a method of controlling an engine speed at the time of idling to a target idling speed, there is a method of changing a throttle valve opening degree and thereby controlling the intake air amount when the engine speed deviates from the target idling speed so that the engine speed becomes the target idling speed. However, in this case, there is a problem in that there is a time delay from when the amount of intake air supplied in an engine cylinder changes until the generated torque of the engine changes, so the engine speed cannot be made to easily settle down to the target idling speed.
In contrast, the generated torque of an engine changes immediately if changing the ignition timing, so the engine speed may be returned to the target idling speed with good response by controlling the ignition timing so that the engine speed becomes the target idling speed. However, there is a limit to the amount of change of a generated torque of an engine by changing the ignition timing. The generated torque of the engine cannot be changed by a large extent as when the intake air amount is changed.
Known in the art is an internal combustion engine designed to simultaneously control the intake air amount and the ignition timing so that when the engine speed deviates from the target idling speed, the engine speed becomes the target idling speed (see for example Japanese Patent Publication (A) No. 64-80752).
On the other hand, when an actual compression ratio changes, the amount of change of the generated torque of the engine when changing the ignition timing greatly changes, and the lower the actual compression ratio, the greater the amount of change of the generated torque of the engine when changing the ignition timing. In this case, if the amount of change of the generated torque of the engine when changing the ignition timing is great, due to the occurrence of hunting, the engine speed will not easily settle down to the target idling speed. If the amount of change of the generated torque of the engine when changing the ignition timing is small, time is needed until the engine speed settles down to the target idling speed. That is, there is an optimal value for the amount of change of the generated torque of the engine when changing the ignition timing.
Accordingly, in a spark ignition type internal combustion engine able to change the actual compression ratio, when changing the actual compression ratio at the time of idling, it is necessary to make the amount of change of the generated torque of the engine when changing the ignition timing become optimum no matter what the actual compression ratio.